The present invention relates to an arrangement for registering the instant volume or the instant level of the charge in an ore-grinding drum of the kind that is provided with internal lifting means.
When processing mineral material for the selective or collective recovery of valuable material components, the processes concerned are preceded by mechanical crushing or disintegration of the material in a manner to free the valuable components, one from the other. The components are then mutually isolated with the aid of known separation methods, this isolation being contingent on differences in colour, shape, density or in differences in their respective surface active and magnetic properties, or other properties.
Mechanical crushing or disintegration of the ore material is normally started when the rock is blasted, and then continues successively in a series of disintegrating operations, which may be of mutually different kinds. The process used normally involves crushing the material in several stages with the aid of jaw crushers and/or cone crushers, with subsequent grinding of the material in rotating drum mills which include grinding bodies in the form of steel balls or steel bars. This conventional grinding of materials, however, results in considerable wear on the grinding bodies present in the mill, due to the hardness of the rock concerned, therewith also resulting in considerable costs for the provision of such grinding bodies.
In order to avoid this, a technique has been developed successfully in which the actual material itself, i.e. the material to be ground, forms the grinding bodies. This technique is known as autogenous grinding and is widely used.
In autogenous grinding systems, the composition of the grinding charge formed in the grinding drum is dependent on the properties of the material concerned. Existing mineral deposits, however, seldom have an homogenous structure and a homogenous mechanical strength, and consequently the autogenous grinding process requires a varying energy input, due to a naturally formed particle size composition of the grinding charge which is unsuitable for grinding purposes and which is known as the "critical size" and implies an over-representation of certain particle size fractions in the grinding charge, due to the incompetence of the material in autogenous grinding processes.
When critical particle sizes are formed in the mill, the mill is no longer able to function in the manner intended and the throughflow of material is quickly impaired, resulting in an increased energy requirement in kwh/tonne of ore, in order to achieve a predetermined degree of grinding.
The energy or power requirement of a mill depends on several factors, such as the density of the grinding charge, a mill constant, the extent of mill charge replenishment, or the instant volume of charge in the mill, relative mill speed, length and diameter of the mill. Normally, the weight of the grinding charge has been used as the deciding parameter for controlling the mill. This method is cost demanding, however, because of the weighing equipment needed to register continuously the changes in the weight of the grinding charge that occur during operation of the mill, which enables the steps necessary in order to improve prevailing operating conditions to be carried out as quickly as possible.